Ion-conducting materials are capable of strongly bonding with certain ions or allowing for selective permeation of certain cations or anions. Because of this nature, they are processed into particulate, fiber or membrane form and utilized in a variety of applications including electrodialysis, diffusion dialysis, and cell diaphragms. For example, polymer electrolyte fuel cells (PEFCs) are constructed such that a fuel such as hydrogen or methanol is electrochemically oxidized in the cell using a polymer electrolyte membrane, whereby the chemical energy of the fuel is directly converted into the electric energy which is extractable out of the cell. The PEFCs are now of great concern as a clean electric energy source. In particular, polymers having functional groups such as sulfonic groups within the polymer chain are expected to be a potential power source material for electric vehicles because they can form proton-conducting membranes with high outputs and low-temperature performance.
Such fuel cells are generally constructed as comprising an electrolyte membrane, gas diffusion electrodes having a pair of catalyst layers joined to the opposed surfaces of the membrane, and current collectors disposed on the opposed surfaces of the electrodes. The fuel such as hydrogen or methanol is fed to one electrode or anode while the oxidant such as oxygen or air is fed to the other electrode or cathode. An external load circuit is connected between the electrodes. During operation of the fuel cell, protons produced on the anode migrate toward the cathode through the electrolyte membrane and react with oxygen on the cathode to form water.
The electrolyte membrane functions as a migratory medium for protons and as a diaphragm for hydrogen and oxygen gases. It is thus required to have a gas seal ability as well as proton conduction, strength and chemical stability. Also in the case of fuel cells and water electrolysis, peroxides form in a catalyst layer formed at the interface between the electrolyte membrane and the electrode. These peroxides diffuse and become radicals, incurring degradation reaction. This prohibits the use of polymer membranes having poor oxidation resistance.
Most of the electrolyte membranes which are practically acceptable as having high oxidation resistance are fluorine-based membranes possessing a main skeleton of perfluoroalkylene and having ion-exchange groups such as sulfonic or carboxylic acid groups at the end of some perfluorovinyl ether side chains. These fluorine-based membranes are known and commercially available as Nafion® membranes from DuPont, Dow® membranes from Dow Chemical, Aciplex® membranes from Asahi Chemical Industry Co., Ltd., and Flemion® membranes from Asahi Glass Co., Ltd.
These membranes have been practically proven in the brine electrolysis industry owing to their stability, but suffer from the following problems because they are fluoroplastics having sulfonic groups.
1. Fluorinated electrolytes having sulfonic groups are difficult to manufacture and thus very expensive. In the attempt of applying PEFCs to vehicles, the cost of Nafion membranes must be reduced to a fraction as low as one several tenths or one several hundredths before they can be commercially accepted.2. As the amount of sulfonic groups is increased in order to reduce the electric resistance, the membrane strength is reduced. A membrane with a low electric resistance suffers from rupture and other problems during cell operation. For this reason, Nafion and equivalent membranes pose a limit to the amount of sulfonic groups which can be incorporated, with the upper limit being an ion-exchange capacity of 1.1 milli-equivalent/gram.3. The existing fluorinated electrolyte membranes having sulfonic groups can be used substantially solely at temperatures below 100° C. This is because in a temperature range higher than the Tg around 120° C. of polymers, the ion channel structure contributing to proton conduction is broken, inhibiting proton conduction through the cluster channels created by water and sulfonic groups in the membrane.4. No proton conduction is provided in the absence of water. Since ionic conductivity largely depends on the water content of the membrane governed by the humidity of the cell service environment, a strict and complex control of the water content of the membrane by humidifying the fuel is necessary. This makes the structure of the fuel cell more intricate and the device larger-sized, posing a greater burden to the device and even causing failures.
Under the circumstances, other sulfonated polymers such as polyimide, polysulfone, polystyrene, polyphenylene, polyether ether ketone (PEEK) and the like were developed as the polymer electrolyte membranes that can replace the fluorinated electrolyte membranes.
However, the post-sulfonation method of forming the above-described sulfonated copolymer membranes has the following problems associated with its sulfonation step.
1. Since a variety of sulfonation agents used in the sulfonation step are hazardous chemicals, the step cannot be devoid of hazards despite careful handling of agents and deliberate designing of the process unit.
2. In order to introduce sulfone groups into a polymer of styrene skeleton, the introduction of sulfone groups must be carried out for a long period of time or under rigorous sulfonating conditions. Then, there inevitably occur side reactions other than the desired sulfonation reaction. For example, in an attempt to introduce siloxy groups, the elimination of siloxy groups and the formation of crosslinks are inevitable. This exacerbates the efficiency of introduction, resulting in a degradation of membrane performance, especially a decline of mechanical strength.3. The sulfonating conditions must be adjusted in order to produce membranes having different ion-exchange capacity. It is quite difficult to strictly control the conditions in a reproducible manner, which is a problem from the standpoint of quality control. There is a demand for a process which can omit the sulfonating step using sulfonating agents.
As discussed above, the currently available electrolytes have many drawbacks and are problematic in that they cannot fully comply with low-humidity/high-temperature operation as encountered in fuel cells or the like. There is thus a desire to have ion-conducting/ion-exchanging materials that can replace the electrolytes. Silicon-based polymers having high oxidation resistance have already been developed.
JP-A 14-184427 discloses a method for preparing a proton-conducting film with heat resistance by forming a crosslinked structure which is a combination of a mercapto group-containing alkoxysilane, boron oxide, and another alkoxysilyl compound, followed by oxidation. With this method, however, the crosslinked structure of a mercapto group-containing alkoxysilane and boron oxide, and the crosslinked structure of a mercapto group-containing alkoxysilane, boron oxide, and another alkoxysilyl compound are available in powder form, and thus these structures alone cannot be formed into a film. Accordingly, for the film formation purpose, these structures must be combined with other polymeric materials. The composite films are not always highly heat resistant for the reason that even though the crosslinked structures themselves are fully heat resistant, the polymeric materials to be combined are less heat resistant.
Also, JP-A 2006-131770 discloses a method for preparing an electroconductive film by forming a crosslinked structure which is a combination of a mercapto group-containing alkoxysilane with another alkoxysilyl compound, followed by oxidation. With this method, however, films having a conductivity of the order that can be evaluated only in terms of surface resistance are merely available. Their application is limited to the field of surface coating. They cannot be used at all in the electrolyte membrane application.
Also described in Solid State Ionics, vol. 74, 105, 1994, is a method for preparing an electrolyte material by combining a mercapto group-containing alkoxysilane with another alkoxysilyl compound, followed by crosslinking and oxidation. Although the material is not specified with respect to its form or the like, it is explicitly described that the material exhibits deliquescence at high humidity, indicating that the material cannot be used as a proton-conducting membrane.
Kaliaguine, Microporous and Mesoporous Materials, vol. 52, 29-37, 2002, reports a method for preparing an electrolyte material having micropores serving as ion channels by mixing a mercapto group-containing alkoxysilane with tetraethoxysilane in a varying ratio, causing the mixture to crosslink in the presence of a surfactant or the like, followed by oxidation. The film obtained by this method, however, does not exert a full effect on the proton conduction at low humidity.
JP-A 2005-89682 discloses a proton-conducting fluoropolyether composition comprising a compound of perfluoropolyether structure, an organosilicon compound having at least two hydrogen atoms, a hydrosilylation catalyst, and a proton conductive agent. However, the proton conductive agent is limited to heteropoly-acids in a substantial sense, and with other agents, crosslinking does not take place to such an extent that a membrane formed therefrom has satisfactory strength.